Apparatus for encapsulation casting

ABSTRACT

An apparatus and a method of use thereof for cast encapsulation of items, particularly electronic components. The apparatus consists of a plurality of identically shaped split matrix elements. Each matrix element has a first surface for receiving and securing the item to be encapsulated. A second surface of another identically shaped matrix element, located on a side thereof opposite to its first surface, is then mated with the first surface securing the item. Thus mated, the first and second surfaces establish a molding cavity for enclosing the item and an orifice providing access thereto. This process of securing an item to a first surface and enclosing it with a second surface is repeated thereby assembling a stack of matrix elements. This stack is then rigidly secured and the molding cavities are filled with particulate filler material through upright orifices. Excess filler is removed by quickly turning the stack over and then righting it again. The stack of matrices is then heated, its orifices filled with a quantity of heated, liquid thermosetting encapsulating compound and exposed to vacuum whereby substantially all air is drawn from the molding cavities through the liquid encapsulating compound. The stack is then again exposed to atmospheric pressure forcing the encapsulating compound throughout the unoccupied voids in the molding cavities after which that compound is permitted to solidify, thereby completing the encapsulation process.

This is a division, of application Ser. No. 06/224,658, filed Jan. 13,1981 now U.S. Pat. No. 4,374,080.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to casting and more particularlyto encapsulation of items within thermosetting polymeric materials.

2. Description of the Prior Art

Cast encapsulation of items, particularly electronic components, is awell established packaging technology. Currently, such items areencapsulated by a process called transfer molding. In this process, athermosetting encapsulation material is retained within a chamber in atransfer molding machine while an item to be encapsulated is securedwithin a molding cavity in another portion of the machine. The chambercontaining the thermosetting encapsulant and the molding cavity areconnected by one or more passages so that pressure applied to thethermosetting encapsulant may cause it to flow into the molding cavityand around the item to be encapsulated. After the thermosettingencapsulant has solidified, the item, now encapsulated, may be removedfrom the molding cavity.

The encapsulation material often used in the transfer molding process isan epoxy resin mixed with a particulate filler such as silica oralumina. These particulate materials are incorporated into theencapsulation material principally to increase its thermal conductivity.The greater the concentration of particulate filler in the encapsulationmaterial, the higher its thermal conductivity. However, addition of thisparticulate material to the thermosetting resin also increases itsviscosity, thereby creating a corresponding increase in the forcerequired for transfer molding.

There are several problems with the transfer molding process ascurrently employed to encapsulate items such as electronic components.First, because high pressures are required to transfer the thermosettingencapsulant into the molding cavity, that cavity must be fabricated frommetal. Since the thermosetting encapsulant materials such as epoxyresins normally bond well to metal, the general practice in the industryis to incorporate a mold release compound into the encapsulant materialso that after it has solidified the encapsulated item may be easilyremoved from the molding cavity. However, since the electricallyconductive leads by which electrical currents flow into or out of anelectronic component are generally made of metal, these mold releasecompounds may also permit the encapsulation to separate from theelectrically conductive leads. Since contact with air, particularly themoisture in air, often degrades the performance of electroniccomponents, the detachment of the encapsulation from the electricallyconductive leads provides a path by which moisture may reach anencapsulated component and thus may contribute to its failure.

A second problem with transfer molding as currently practiced inencapsulating electronic components is referred to as "wire sweep." Theterm "wire sweep" describes the bending of electrically conductive leadsby the force of inflowing encapsulation material. Since the electricallyconductive leads of electronic components being encapsulated aregenerally quite fragile and must remain electrically insulated from eachother after encapsulation, "wire sweep" establishes an upper limit onthe rate at which encapsulation material may be transferred into themolding cavity. Furthermore, this rate of transfer decreases as theviscosity of the encapsulation material increases such as occurs withincreasing concentrations of particulate filler. Therefore, the currenttransfer molding process involves a trade-off between the thermalconductivity of the encapsulation material and the speed with which themolding cavity may be filled without damaging the item beingencapsulated.

SUMMARY OF THE PRESENT INVENTION

An object of the present invention is to provide an encapsulationprocess employing simple and economical apparatus therefor.

Another object of the present invention is to provide an encapsulationfor electronic components having high thermal conductivity.

Another object of the present invention is to provide an encapsulationfor electronic components in which the volume of the encapsulation isfully occupied by particulate filler.

Another object of the present invention is to provide an encapsulationwhich excludes moisture therefrom.

A further object of the present invention is to provide an encapsulationprocess which does not require the inclusion of mold release compoundsinto the encapsulation material.

Briefly, the apparatus employed in the preferred embodiment of thepresent invention includes a plurality of identically shaped splitmatrix elements. Each matrix element has a first surface shaped to forma portion of a molding cavity surface and to receive and retain the itemto be encapsulated. A second surface of each matrix element is shaped tomate with the first surface thereof of another element and, when somated, to establish the remainder of a molding cavity surface enclosingthe item to be encapsulated. The first and second surfaces of eachmatrix element are located on opposite sides thereof relative to oneanother so that, in principal, an unlimited number of elements may besuccessively mated to assemble a stack for enclosing items to beencapsulated. The surfaces of the matrix elements are also shaped so asto establish an orifice leading from the exterior to the molding cavityby which encapsulation material may be introduced thereinto.

In the preferred embodiment of this invention, these identically shapedmatrix elements are cast from silicone rubber. Since thermosettngencapsulation materials, particularly epoxies, do not adhere to siliconerubber, mold release agents need not be included in the encapsulationmaterial cast with these matrix elements.

A stack of the matrix elements is established by successively securingan item to be encapsulated to a first surface of a first matrix elementand then enclosing the item within a molding cavity established bymating the second surface of a second matrix element thereto. The stackmay be increased by including third, fourth, fifth, etc., elements in astacking relationship. The stack thus assembled is rigidly secured andpositioned so that the exterior opening to the orifices leading to themolding cavities face upward. The particulate filler to be incorporatedinto the encapsulation may then be introduced into these orifices sothat it may fill the molding cavities much in the same way that sandflows from the top to the bottom of an hour glass. The manner in whichthe particulate filler material enters the molding cavity is a gentleone and each cavity becomes filled with the maximum possible amount ofparticulate filler. Furthermore, the particulate material thus fillingthe molding cavities mechanically supports the electrically conductiveleads, thus preventing their damage by subsequent introduction of othermaterial thereinto. In the preferred embodiment, the orifices forfilling the molding cavities are shaped so as to have a narrow gateregion immediately adjacent to the cavity and a much broader sprueregion further therefrom. This shape permits removal of any excessparticulate filler from these orifices by a quick, 360 degree rotationof the rigidly secured matrix element stack. During such a rotation,particulate material easily flows from the orifice while it is retainedwithin the molding cavity by the narrow gate region thereof.

The rigidly secured stack of matrix elements establishing moldingcavities now containing an item to be encapsulated surrounded byparticulate filler material is then heated and its upright orifices arefilled with a quantity of heated, liquid thermosetting encapsulatingcompound. This assembly is then exposed to vacuum which drawssubstantially all air including all moisture from the molding cavities.These gasses leave the molding cavities by bubbling up through theliquid encapsulating compound in the orifices leading thereto. After allgasses have been removed from the volume of the molding cavityunoccupied by the particulate filler, the assembly is again exposed toatmospheric pressure which forces the encapsulating compound throughoutthe molding cavity. The liquid encapsulating compound is then permittedto solidify, after which the stack of matrix elements is disassembledand the encapsulated items are removed from the molds. Since the liquidencapsulation material employed in the preferred embodiment of thisinvention does not include a mold release agent, the encapsulation thusformed integrally bonds to the electrically conductive leads connectedto an encapsulated electronic component, thereby establishing a barrierto future entry of moisture into the encapsulation.

An advantage of the process of the present invention is that it employsa simple and economical molding apparatus.

Another advantage is that an encapsulation fabricated in accordance withthis invention has excellent thermal conductivity.

Yet another advantage is that an encapsulation has the highest possiblevolume thereof occupied by particulate filler material.

Still another advantage of the present invention is that moisture isremoved from the encapsulated item during the process.

A further advantage of the present invention is that the thermosettingresin need not include a mold release compound.

These and other objects and advantages of the present invention will nodoubt become apparent to those skilled in the art after having read thefollowing detailed description of the preferred embodiment which isillustrated in the several figures of the drawings.

IN THE DRAWINGS

FIG. 1 is a partially sectioned perspective view of a stack assembly ofmatrix elements enclosing items to be encapsulated in accordance withthe present invention;

FIG. 1A is a bottom view of a section of another element which may beplaced on top of the top element of FIG. 1 to further increase thestack;

FIG. 2 is a cross-sectional view of the stack of matrix elements takenalong the line 2--2 of FIG. 1, with the element of FIG. 1A stackedthereon;

FIG. 3 is a cross-sectional view of the stack of matrix elements takenalong the line 3--3 of FIG. 1 with the element of FIG. 1A stackedthereon showing particulate filler material being introduced into themolding cavities;

FIG. 4 is a cross-sectional view of the stack of matrix elements of FIG.3 showing evacuation of the molding cavities;

FIG. 5 is a cross-sectional view of the stack of matrix elements of FIG.4 showing solidified thermosetting resin; and

FIG. 6 is a perspective view of an encapsulated item after encapsulationand removal from the molding cavities of the assembly of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 illustrates a plurality of split matrix elements, each referredto by the general reference number 10, being assembled on top of oneanother to form an overall stack 12. The stack 12 is adapted to receiveand enclose a plurality of electronic component assemblies referred toby the general reference number 20, to be encapsulated. The illustratedelectronic component assemblies 20 are in the form of Dual InclinePackage lead frames 22, (DIP lead frame) each of which includes aplurality of electronic component chips 24. Each DIP lead frame 22includes a plurality of pin region openings 26, periodically spacedalong its length. A reference aperture 28 is formed into bothlongitudinal edges of the frame adjacent each pin region 26. The DIPlead frame 22 further includes plurality of chip bonding region opening30 also spaced periodically along its length between the pin regionopening 26 and respectively separated therefrom by dam bars 32.

Each split matrix element 10 forms a first longitudinal surface 40 witha depressed frame region 42 (see FIG. 1), adapted to receive the leadframe 22 formed about the center. Along one longitudinal outer edge ofthe frame region 42 is a raised lower seal region 44, while along theother longitudinal outer edge is a raised orifice forming region 46.Formed into the frame region 42 at periodic intervals along its lengthare a plurality of halves of a molding cavity surface 48 in the form ofa rectangular depression. The mold cavity surfaces 48 are laterallyspaced along the length of the frame region 42 and are shaped to matewith the chip bonding regions 30 of the lead frame 22. Also formed intothe frame region 42 intermediate adjacent molding cavity surfaces 48 andadjacent to the orifice region 46 are pin apertures 50 located so as tomate with the reference apertures 28 of the lead frames 22 to beencapsulated. After the lead frame 22 has been positioned within theframe region 42, a pin 52 is inserted through the reference aperture 28and into the aperture 50 to assure alignment of the dam bars 32 with theedges of the molding cavity surface 48 (see FIGS. 1 and 2). Also formedintegrally into the split matrix element 10 and projecting from thelower sealing region 44 are reference pins 54.

Each of the identically shaped matrix elements 10 is formed with asecond surface 60, shown in FIG. 1A, located opposite to the firstsurface 40. The second surface 60 is essentially planar, having formedthereinto a plurality of periodically spaced molding cavities 62. Themolding cavities 62 are also spaced along the length of the secondsurface 60 and are also shaped so as to mate with the chip bondingopening regions 30 of the lead frame 22. Along one edge of the surface60 at the end of each of the molding cavity surfaces 62 is an orificeregion 64 extending to the exterior of the element 10. Each orificeregion 64 has a gate segment 66 immediately adjacent to the moldingcavity 62 and a sprue segment 68 (lying between adjacent pairs of dashedlines 69) farthest therefrom and extending to the exterior. In theembodiment 10, the sprue segments 68 of each orifice region 64 areinterconnected to facilitate simultaneous filling of the moldingcavities 62. Also formed into the second surface 60 along thelongitudinal edge farthest from the orifice region 64 are referenceapertures 70. The reference apertures 70 mate with the reference pins 54to align the molding cavitys 62 in the second surface 60 with themolding cavitys 48 of the first surface 40. In the preferred embodiment,the split matrix elements 10 are cast from a silicone rubber materialhaving a hardness of approximately sixty durometers when solidified.

As shown in FIG. 1 and FIG. 1A, the stack assembly 12 of matrix elements10 is assembled by locating an electronic component assembly 20 withinthe frame region 42 of a split matrix element 10 and securing it theretoby inserting a pin 52 through the reference aperture 28 in the leadframe 22 and into the pin aperture 50 (See FIG. 2). With the electroniccomponent assembly 20 thus secured about the first surface 40 of a splitmatrix element 10, the next layer in the stack 12 is assembled by matingthe second surface 60 of another of the matrix elements 10 to the firstsurface 40. Thus, in principle, a stack assembly 12 may include anunlimited number of split matrix elements 10. Thus, an unlimited numberof components 20 may be simultaneously accommodated with each electroniccomponent assembly 20 located intermediate mated first surfaces 40 andsecond surfaces 60. Also, the elements 10 may be of unlimited length toaccommodate a plurality of lead frames 22 in tandem.

The stack 12 of split matrix elements 10 enclosing electronic componentassemblies 20 thus assembled may then be rigidly secured by applying aclamping force to the exposed outer first surface 40 and exposed secondsurface 60. This is illustrated by the opposing arrows on opposite sidesof the stack 12 in FIG. 2. Thus secured within the stack 12, the pinregion openings 26 of the lead frames 22 become isolated from the chipbonding region openings 30 by the dam bars 32 which are clamped securelyalong their entire length between the mated surfaces 40 and 60.

As shown in FIG. 3, the mated first surfaces 40 and second surfaces 60establish a plurality of composite molding cavities 80 about the chipbonding region 30 of the lead frame 22 and the electronic component chip24 bonded thereto. The mated surfaces 40 and 60 also establish an upwarddirected orifice 82 extending from the molding cavity 80. The gatesegment 66 of the orifice region 64 narrows the orifice 82 at the pointof entry to the molding cavity 80. This narrowing of the orifice 82serves to limit both the rate at which material may enter the moldingcavity 80 and the rate at which such material may leave if the stack 12is inverted. The widened orifice 82 at the sprue segment 68 serves as areservoir for material entering the molding cavity 80.

After the stack 12 is secured, particulate filler material 90 such assilica or alumina may then be introduced into the molding cavities 80through their associated orifices 82. The particulate material 90 entersthe molding cavity 80 slowly and gently as a falling stream due to thenarrowing of the orifice 82 at the gate segment 66 thus avoiding "wiresweep." Excess particulate material 90 remaining in the orifices 82after the molding cavities 80 have been filled may be removed therefromby quickly turning the stack 12 over and then righting it again.

The prepared stack 12 is then heated. With lead frame, this is generallyto approximately 150 degrees centigrade. Then a quantity of liquid,thermosetting epoxy resin 92 not containing a mold release compound andheated to the same temperature is introduced into the orifices 82 asshown in FIG. 4. The stack 12 is then exposed to vacuum so that all theair and, in particular, the moisture in the voids between theparticulate filler material 90 in the molding cavities 80 may be removedtherefrom. Since rigidly securing the stack 12 has sealed the leadframes 22 between the surfaces 40 and 60 everywhere but at the orifices82, these gasses escape from the molding cavities 80 by bubbling upthrough the liquified epoxy resin 92. Once the molding cavities 80 hasbeen evacuated, the stack 12 is returned to atmospheric pressure therebyforcing the liquid thermosetting resin 92 throughout the moldingcavities 80. Because the molding cavities 80 has been previously filledwith the particulate filler material 92, this introduction of the liquidthermosetting resin 92 cannot cause "wire sweep." Further, the liquidthermosetting resin 92 cannot reach the pin regions 26 because they areisolated from the chip bonding regions 30 by the dam bars 32 clampedsecurely along their entire length between the mated surfaces 40 and 60.The thermosetting resin 92 is then permitted to solidify within themolding cavities 80 of the rigidly secured stack 12. Thus solidified,the particulate material 90 and the thermosetting resin 92 become anencapsulation 94 about the electronic component chip 24.

An alternative procedure for introducing the thermosetting epoxy resin92 into the molding cavities 80 is to expose the heated stack 12 tovacuum, introduce the heated thermosetting epoxy resin 92 into theorifices 82 while maintaining the stack 12 under vacuum and thenreturning the heated and filled stack 12 to atmospheric pressure. Anadvantage obtained with this alternative filling procedure is thatmoisture within the cavity 80 does not escape by passing through theepoxy resin 92 thus eliminating any possibility of further increasingthe moisture content of the resin 92.

The stack 12 may now be disassembled to remove the encapsulatedelectronic component assembly 20. This disassembly is easilyaccomplished since the thermosetting epoxy resin 92 does not adhere tothe silicone rubber from which the split matrix elements 10 are cast.However, since the resin 92 does not contain a mold release compound,the encapsulation 94 adheres strongly to the DIP lead frame 22. Afterremoval from the stack 12, any excess resin 94 which remained in theorifice 82 during solidification is easily broken free from theencapsulation 94 at the narrow neck region formed therein by the gatesegment 66, thus producing the encapsulated electronic componentassembly shown in FIG. 6.

Although the present invention has been described in terms of thepresently preferred embodiment, in particular encapsulating electroniccomponents secured to DIP lead frames, it is to be understood that suchdisclosure is not to be interpreted as limiting. Various alterations andmodifications will no doubt become apparent to those skilled in the artafter having read the above disclosure. Accordingly, it is intended thatthe appended claims be interpreted as covering all alterations andmodifications as fall within the true spirit and scope of the invention.

What is claimed is:
 1. A unitary matrix element for cast encapsulationof an item comprising:a first surface shaped and adapted to receive andsecure an item to be encapsulated, the first surface being shaped toestablish a portion of a molding cavity for enclosing an item to beencapsulated and a portion of an orifice surface extending from saidmolding cavity surface; and a second surface located on an opposite sideof the matrix element from the first surface, the second surface beingshaped to mate with the first surface of another similar matrix elementand to establish the remainder of said molding cavity surface and saidorifice surface when two of such matrix elements are stacked together.2. The matrix element of claim 1 further comprisingregistration meansformed on the first surface of the matrix element for securing the itemto be encapsulated thereto.
 3. The matrix element of claim 1 furthercomprisingmating registration means formed on the first and secondsurfaces of the matrix element for securing the elements in their matedrelationship.
 4. The matrix element of claim 1 whereinthe element iscast from silicone rubber.
 5. The matrix element of claim 4 whereintheelement cast from silicon rubber has a hardness between 40 and 90durometers.
 6. The matrix element of claim 1 whereinthe first surface isshaped to receive and secure a DIP lead frame having a chip bondingregion and a pin region which are separated by a dam bar; and the firstand second surfaces of the matrix element are adapted to contact saiddam bar along the entire length thereof; whereby mated first and secondsurfaces of a pair of matrix elements enclosing a received and securedDIP lead frame isolate said chip bonding region thereof from said pinregion thereof.